The private sector example of Fedex does deliver on Saturday... but it is specially handled, and invokes extra charges.
The USPS should make this decision on its own, as a business decision, and not encumbered by the political whims of congress.
Problem is the USPS doesn’t get to make any business decision unencumbered of the political whims of congress. The primary reason they’re in trouble now is because congress makes them pay for the next 50 years of their pension, if they didn’t have to do that they wouldn’t be losing money and none of the things they’re doing to cut costs would be necessary.